Various forms of rocket and similar propulsion motors exist, the primary ones comprising solid fuel and liquid fuel motors which use a chemical reaction between two or more materials to produce a jet of matter for propulsion. The present motors are complex, expensive, and dangerous. The cost of putting a payload into low earth orbit is somewhere between $3000 and $6000 per Kg. In addition to the high cost, the availability of launches is restricted.
The number of man-hours in space continues to increase and consequently the amount of payload in orbit increases. The number of payloads per year increases and also the weight per payload. The capability of launches is increasing but demand rises faster.
It has been proposed to build a propellant plant on the moon to extend the capacity of launcher and possibly reduce costs of space usage. It takes approximately one-tenth of the energy to bring material from the moon to low earth orbit in comparison to the energy required from the surface of the earth. Producing such propellant on the moon could be expensive.